Zinc chromate



Patented Aug. 5, 1941 Reuben W. Iieisy, Palmerton, Pa., assignor to TheNew Jersey Zinc Company, New Ijork, N. Y., a, corporation of New JerseyNo Drawing. Application April 29, 1940, Serial No. 382,267

,7 Claims.

This invention relates to zinc chromate. It provides a basic zincchromate of novel composition and superior properties for use ,incoating compositions and methods for producing such:

zinc chromate.

It has been proposed heretofore to employ zinc,

chromates as pigments, but the zinc chromates or "zinc yellowsheretofore available for such purpose have been characterized by arelatively low ratio of ZnO content to CrO; content in comparison withthe basic zinc chromate of the present invention and have contained arelatively large proportion of water-soluble constituents which tend toproduce blistering of paint films in which the pigment zinc chromate isincorporated. Thus, some of the zinc chromate content of such pigmentshas been water-soluble and the pigments have also contained a relativelylarge proportion of other water-soluble salts. Zinc chromate pigmentsavailable heretofore contain 36% toA% ZnQ', 36% to 48% CrOs.

and 18% to 34% water-soluble salts. These are,

in addition to zinc chromate and other chromates,

tice. In the table, the first column represents the distance in Angstromunits between crystal planes from which X-rays are reflected with arelative intensity given inthe second column. In

the second column vs represents very strong intensity, "8 strong, "mmoderate, 10 weak, and f faint.

10 Relative Spacing intensi ties A. nits 4.1;; m l- 3. s 3.25 w 2. 73 s2.55 vs 2.18 m 1.811 m 1.754 10 1. s l. m 1.454- f 1.371 f 1.340 101.270 f 1.190 f compoundsof zinc, sodium, or potassium with radicalssuch as sulfate, which compounds result from the-method .of manufactureof the pigments, i. e., reaction of sodium or potassium dichromates withzinc sulfate or wlth zinc oxide plussulfuric acid. 1

Zinc chromate pigments heretofore available arebright yellow in the drystate and tend tb impart such a color to paint films in which they areemployed.'. However, it has been necessary to employ expensivesyntheticresin waterproof ve-' hicles with these pigments in order toproduce films that have resistance to blistering;

As a result of my investigations-, I have produced a zinc chromate ofnovel comDO tlon and. superior properties. "The nine chromate of myinvention is basic and contains a higher ratio'of The 'zinc chromate ormy invention contains 5 mols of ZnO per mol of CrOz. composition inparts by weight is 70.3% ZnO,

11.3% 010;, 57nd 12.4% mo, and (as disclosed in detail hereinafter) itmay be prepared in substantially pure state with a content ofwatersoluble salts that is less than 1/100 of the watersoluble saltcontent of heretofore customary zinc chromate pigments. f

In its dry powdered state, the zinc chromate pigment of my invention isbright yellow, but it imparts td an oil film a yellowish-brown color,

the shade of which varies substantially with the conditions prevailingduring the manufacture of 40 the pigment. As a pigment therefore, theprodzinc content to chromium content-than do zinc chromates knownheretofore. Thus, the basic zinc chromate of my invention has theformula 5ZnO.CrO$.4H-z0, which may also be written ZnCrO4.4Zn(0H) 2, issubstantially water-insoluble and manifests the fact that it, is achemical individual by several other unique chemical and physicalproperties including an X-ray pattern diflerent from that of theso-called zinc chromate orv zinc yellows heretofore available. Moreover,the X-ray pattern of m0 is absent from, the compound. 1 The compound ofmy invention may be, identified by its X-ray pattern, whichindicates'the following interplanar spacings fof the crystal latuctfinds its principal application in primer paints employed upon metals,such as iron, steel or galvanized iron, rather thanin paints for topcoats which ordinarily sh'ouldbe'of more attractive color; Howeventheproduct has marked rustprotectlve properties and may be employed aloneor mixed with other rust-preventive or fust-in- 'hibiting pigments, suchas zinc oxide, iron oxide,

lead chromatered lead, zinc dust and inerts, in

cases wherev protection ratherithan omamentation is the dominantconsideration. It does not produce blistering of "paint films in whichit is incorporated even when such films are exposed :to water. ,Incontrast with heretofore customary -5 .zin'c chromate pigments whichrequire special Its theoretical vehicles to avoid blistering paintfilms, it can be used in ordinary linseed oil vehicles and in many otherconventional vehicles to produce a waterresistant non-blisteringmetal-protective film.

My presently preferred method of producing 5 the zinc chromate5Z110.Cr0s.4HaO in accord ance with the inventioncomprises directtreatment of zinc oxide in aqueous suspension with a theoretical amountof dissolved chromium trioxide, CrOs, 'i. e., employing ZnO and CrOs inmolal proportions of about 5:1. Such treatment at '-ordinarytemperatures results in a rapid reaction; and if excessive amounts ofwater are not present the mixture sets to a cake that may be driedwithout filtering. After drying, the material requires only to bedisintegrated to form a suitable pigment. The basic zinc chromatesoprepared imparts a high consistency to paint per unit of pigmentincorporated. If desired,

- however, the precipitation-process can-beso con- 2o trolled that afluid suspension rather than a stiff. cake is obtained, this resultbeing brought'about by increasing the amount of water employed, by r 1stirring longer or more violently, or by employing water-are-addedrapidly to the suspension with agitatiom -The reaction is carried out atroom "temperature, say, 30 C. The reacting mass be-- 40 manufacturetion;

higher temperatures. In-the casein which the fluid suspension isproduced, the precipitate of zinc chromate must be separated from themother liquor by clarification treatment, for example, by filtration. V

The following is a practical example of the I of-the zincchromate of my'inven-v v 4 Eztample I l Five hundred kilograms of finely-divided zincoxide are stirredinto 5000 liters of water to form a slurry orsuspension, and 125 kilograms of chromium trioxide CrOs' dissolved in150' liters of comes a- -soli d cake'in about nine minutes. .This cakeis driedto a constant water content, and

thereafter disintegrated. The temperature of The resulting cake containssubstantially 17.3% D I C 03, 70.4%,Zn0, 8nd 12.3% H20, thuscorreblistering.

d y n *nfay vary, but need not exceed about 100 C. 4

sponding very closelyto the theoretical composition of 17.3% CrOa, 70.3%Zn0;12.4% 11:0. The

"content of'water-soluble'salts in the pigment 9 is approximately 0.26%'and this small content is. harmless with respect to the production ofIf desired, mixtures of the are chromate of my invention-with zinc oxidecan be obtained 'by using excess zinc oxide in the process described"above, 1. e., by employing more than five chromate 'of my inventiontend toform soft and porous paint films when incorporated in ordinarypaint vehicles, Generally speaking, the greater the'zinc oxide contentthe softer and moreporous -a soft and porous film is undesirable;'Iprefer o5 molal parts of zinc oxide to one molal part "of CrOa.Such-mixtures of zinc oxide, and the zinc is' the film." Consequently,for purposes where to-produce the zincchromate pigment of theinvention-substantially free from uncombinedzinc oxide. l

The consistency of paints preparedwith the pigment of my invention canbe controlled by appropriate mechanical treatment of the pigment. Thus,the consistency of such paints ordinarily can be "lowered by subjectingthe pigmentto grinding, densiflcation or disintegration beforeincorporating itin the paint. Moreover, the con- Relative spacingintensities 1.1mm 4.19 Moderate '3.63 Strong 3.25 Weak r erystrong 2.18Moderate l.-811 Moderate 1.754 eak 1.574 Strong 1.522 Moderate 1.454Faint 1.371 Faint 1,340 Weak 1.276 Faint 1.190 Faint said pigmentsistency of paints prepared with the pigment of by reacting an alkalimetal dichromate, say. so-

dium or potassium dichromate with 'zinc sulfate in the presence of zinc,oxide, the CIO; content of the chromate being slightly in excess of thatrequired to combine with the zinc present in the ratio 01 1 mol of CrOato 5 mols of ZnO. Thus,

. a product containing a large proportion of the basic zinc chromate ofmy invention can be ob- .tained by operating as follows:

Example II I T Thirty-two. pounds of American process zinc oxide isputinto a tank with approximately 50 gallons of filtered industrialwater and is agitated for ten to fifteen minutes to insure thoroughdispersion of the zinc oxide in the water. To the resulting slurry, 18.6liters ZI1S04 solution (110 grams per liter Zn) is added. file slurry isagitated vigorously during the time in 'fwhich the zinc sulfate isaddedand agitation is continued for forty-five minutes.

At this point, the slurry is heated to a temperature of about 50 C. and22.5-'pounds of sodium dichromate (NazCrzOmZHzO) dissolved in tengallons of wateradded at the-rate of one gallon per minute. The amountof sodium dichromate thusadded is about 10% in. excess of theoretical.Agitation of the slurry is continued during the period in whichthesodium dichromate is added but thereafter-is stopped and the slurry isallowed to stand overnight. Thereafter,

, the slurry is put through a filter press and the resulting cake, afterdrying at 0., is disintegrated to produce the pigment.

The composition of the product according to Example 11 is 65.7% ZnO,.and 19.6%. Cr03.

- X-ray examination of the product indicated the 5 presence of thecompound 5ZnO.Cr0s.4HzO together with other more water-solublecompounds.

-. I claim-.-

1. YA basic zinc chromate having a crystal structure characterized bythe following X-ray spacings:

s 2. A pigment consisting essentially of a zinc chromate having theformula szn'ocroeineo basic soluble salts.

3. A basic zinc chromate containing in parts being substantially free ofwaterby weight was '1o.s% zno, about 113% G103, and about 12.4% H10.

4.. The process of making basic zinc chromate I which comprises treatingzinc oxide in aqueous suspension with a solution of chromium trioxide,the molalratio of zinc oxide present to chromium trioxide present beingnot less than about 5:1. I r

5. The-process of making basiczinc chromate which comprises treatingzinc oxide in an aqueous suspension with dissolved chromium trioxide,the molal ratio of zinc oxide to chromium trioxide present being about5:1.

6. The process of making basic zinc chromate SZnQCrOaAI-IzO whichcomprises incorporating in a slurry of zinc oxidein aqueous zinc sulfate3 which comprises treating zinc oxide inaqueous suspension withdissolved chromium trioxide, the amount of water present being such thatthe resulting. compound -5ZnO,CrOa.4H-z0 sets to a firm moist cake, anddrying said cake.

. 'l. The process of making basic zinc chromate solution dissolvedsodium dichromate,--the CrOa content oi! which is slightly in excess of'that required to combine with the zinc present in the ratio of 1 mol ofCrOrper 5 ;mols of ZnO. V

REUBEN w. LEISY.

